I found the stock seat highly uncomfortable for extended rides too. The shape was wrong for me, and the foam slightly too soft so it overcompressed. Last winter I shipped the seat to Renazco Racing, and the seat I got back is excellent.
You can see it in this pic -- the profile is flatter front to back (I can actually scoot forward and back a little, which I never could on the stock seat due to the dished shape), and the foam is firmer and doesn't overcompress:
Just be advised that Renazco has a long waiting list. But the seats are worth the wait.